Effects of diets containing gossypol on reproductive capacity of rainbow trout (Oncorhynchus mykiss)

We evaluated five practical diets in which 0%, 25%, 50%, 75%, and 100% (die tary treatments 1-5) of fish meal protein was replaced by solvent-extracted cottonseed meal protein. Adult rainbow trout (initial average weight 247 /- 8 g) were fed the diets over a period of 131 days during which a general 2-fold body weight increase occurred. The total diet gossypol concentratio n (free and protein-bound) showed a gradual increase with increased cottons eed meal substitution. Blood samples were collected on Days 0, 64, 112, and 131 for hematological and steroid hormone determination in plasma of males and females. Hemoglobin content was significantly reduced in fish from tre atment 5 (7.9 +/- 0.3 g/dl) in comparison to treatments 1-3 (10.3-10.9 g/dl ). After 112 and 131 days of feeding, testis weights, concentrations of tes tosterone, and 11-ketotestosterone were elevated in fish from dietary treat ments 2 and 3 in comparison to control and diets 4 and 5. On Day 71, sperm were collected from 6 fish per dietary treatment to assess sperm quality. N o significant differences in sperm concentrations (7.2-9.8 x 10(9)/ml), mot ility (78-89%), and standardized (300 x 10(5) sperm/egg) fertilizing abilit y (18.9-22.6% hatched embryos) were found. Total gossypol concentrations in blood plasma differed significantly among treatments, and the levels were among the highest ever recorded in animals fed cottonseed-supplemented diet s (2.9 +/- 0.2, 11.7 +/- 4.1, 21.7 +/- 1.4, and 29.9 +/- 3.9 mu g/ml, for t reatments 2-5, respectively). The major portion of gossypol in blood plasma was protein-bound (81-93%). This was in contrast to minute amounts of goss ypol present in seminal plasma, mostly in free form (0.02-0.18 mu g/ml), wh ich indicates the presence of a barrier between general circulation and the testis with respect to gossypol distribution in lower vertebrates. Thus, t he reproductive parameters of male rainbow trout examined in this study wer e not significantly affected by feeding cottonseed meal for 131 days.